1. Field of the Invention
This invention relates to a wideband amplifier and particularly to a wideband amplifier of a base-grounded type which is, for instance, utilized in picture control for color display in high resolution CRT systems.
2. Description of the Background Art
FIG. 1 illustrates a conventional wideband amplifier of a base-grounded type. The conventional wideband amplifier includes a single NPN transistor Q, a base of which is grounded through a constant voltage supply 1 (cell). A collector of the transistor Q is connected through a load resistor R.sub.L to a power supply V.sub.cc. A value of the collector current is substantially equal to that of the emitter current. In this case, a value of the base current is too small to be taken into account. Accordingly, a value of a signal current supplied from a signal source S connected to DC voltage source BA through an input resistor R.sub.IN to the emitter of the transistor Q is represented as that of the collector current of the transistor Q. An output amplitude obtained at an output terminal T.sub.OUT is determined by values of the load resistor R.sub.L and the signal current supplied to the emitter. In the case when an amplitude of a signal voltage generated at both ends of an input resistor R.sub.IN on the basis of an input signal supplied through an input terminal T.sub.IN is represented as V.sub.IN, then an input signal current I.sub.IN is transmitted to the transistor Q through the input resistor R.sub.IN, which is given by the following expression: I.sub.IN =V.sub.IN /R.sub.IN. Further, an output signal voltage amplitude V.sub.OUT generated at the output terminal T.sub.OUT is given by the following expression: V.sub.OUT =I.sub.IN .times.R.sub.L =V.sub.IN /R.sub.IN .times.R.sub.L. A gain G of the conventional wideband amplifier is given by G=V.sub.OUT /V.sub.IN =R.sub.L /R.sub.IN. Therefore, a larger gain is obtained as a value of the load resistor R. becomes larger than that of the input resistor R.sub.IN.
In the conventional wideband amplifier as mentioned above, the gain G is defined as: EQU G=R.sub.L /R.sub.In ( 1)
where a value of resistance in the emitter of the transistor Q is not taken into account.
A cut-off frequency fc at which the gain is -3dB is defined as the following expression: ##EQU1## where the input resistor, the load resistor, and an output impedance of the transistor Q are respectively represented as R.sub.IN, R.sub.L, and r.sub.o. Parasitic capacitance, namely base-emitter capacitance, base-collector capacitance, and collector-substrate capacitance, are respectively represented as C.sub..pi., C.sub.U, and C.sub.CS.
FIG. 2 shows a schematic sectional configuration of the NPN transistor. As illustrated in FIG. 2, the collectorsubstrate capacitance C.sub.CS is generated at P-N junctions on which a P layer of a substrate SUB is connected to an N layer of the collector C, and having an N.sup.+ layer embedded therein, respectively. In addition, the base-collector capacitance C.sub.U is generated on a P-N junction on which a P layer of a base B is connected to the N layer of the collector C. The collector-substrate capacitance C.sub.CS is larger than the base-collector capacitance C.sub.U. Namely, it is expressed as C.sub.U &lt;&lt;C.sub.CS.
Further, since a value r.sub.o of the output impedance is usually high and larger than the value R.sub.L, namely R.sub.L &lt;&lt;r.sub.o, the expression (2) is represented as the following approximation. ##EQU2##
As is understood from the aforementioned expressions (1) and (2) lector-substrate capacitance C.sub.CS or the values of the input resistor R.sub.IN and the load resistor R.sub.L should be reduced in order to improve the frequency characteristic fc of the conventional wideband amplifier without any change in the gain G.
However, in view of the structure of the NPN transistor, a P-N junction and collector-substrate capacitance C.sub.CS are inevitably produced, as shown in FIG. 2. A value of the capacitance C.sub.CS generated at the P-N junction is the result of a manufacturing technique utilized for semiconductor devices. At the present time, a satisfactory technique by which the capacitance is reduced to a negligible level has not been developed.
Moreover, when the frequency characteristic fc is increased without any change in the gain G by reducing the input resistance R.sub.IN and the load resistance R.sub.L, a dynamic range for an input signal S.sub.1 becomes narrower due to the reduction in the input resistance R.sub.IN of the input resistor. This is because the dynamic range is determined depending upon the input resistor R.sub.IN and a constant current Io flowing between the collector C and the emitter E. A relationship among an input current .DELTA..sub.i generated by the input signal S.sub.1, an amplitude v.sub.in of the input signal, and the input resistor R.sub.IN is represented by the following expression: EQU v.sub.in= R.sub.IN.multidot..DELTA. i
Further, a relationship between the input resistor R.sub.IN, the current Io, and an input dynamic range V.sub.IN is represented by the following expression: EQU Io.multidot.R.sub.IN&gt; V.sub.IN ( 4)
where Io&gt;ON (conducting state).
As is understood from the expression (4), the input dynamic range V.sub.IN is proportionate to the input resistor R.sub.IN and the current Io. Accordingly, as the input resistor R.sub.IN decreases, the input dynamic range V.sub.IN becomes narrower. Therefore, an output dynamic range becomes narrower on the basis of the aforementioned expression (1).
In order to recover the dynamic range which is narrowed due to reduction in the input resistance R.sub.IN and the load resistance R.sub.L, the current Io should be increased.
FIG. 3 shows a relationship between a collector current for one NPN transistor for a semiconductor device such as an IC, and a transition frequency f.sub.T at which a current amplification degree h.sub.FE indicating a ratio of the collector current to an emitter current in an emitter-grounded transistor is 1. As shown in FIG. 3, as the collector current exceeds a value Io' where the transition frequency reaches a value f.sub.TMAX, the transition frequency f.sub.T begins an abrupt decrease. In addition to the collectorsubstrate capacitance C.sub.CS, a maximum allowable current (io) is also determined by the manufacturing limitations.
In addition, it is desirable to reduce a total amount of a consumed current for the IC package by reducing an amount of current flowing in each of the transistors contained in an IC package, in order to conserve electric power. If the amount of current in each transistor is not reduced, the total amount of the consumed electric power for the IC package increases and causes an increase in the temperature of the IC chip. In the worst case, the temperature increases to a level which is unacceptable for IC packages.
As a result, conventional wideband amplifiers are limited in the degree of frequency characteristic fc improvement attainable, without a change in either the gain G and/or the dynamic range.
Referring to FIG. 4, a cascade inversion amplifier of an emitter-grounded type is shown as a typical example of a conventional wideband amplifier. In this amplifier, the frequency characteristic is deteriorated at the input stage where the transistor receives an input signal, due to parasitic capacitance and an input impedance which both have an undesirable influence thereon. Further, the cascade amplifier has an essential problem in that the output dynamic range becomes small due to its structure.